This invention relates to pneumatic tires and primarily to tires for vehicles used in cross-country applications.
Typical vehicles used in cross-country applications often have four wheel drive and are fitted with tires that have prominent tread patterns to provide traction in mud and sand. As used herein "cross-country" refers to off-the-highway operation of vehicles.
Typical cross-country tires are directional to provide good grip and self cleaning properties going forwards but this results in poor traction and self cleaning when the tires rotate in reverse. Tread patterns for cross-country tires are typically bar type treads, or block type treads that give good traction but compromise on lateral stability and when used on highway applications can be noisy and give an uncomfortable ride. Other tread patterns, for example as shown in British Patent Application No. 2 042 992, have been developed having a mid circumferential zig-zag rib with blocks located in the edge portion of the tread on either side of the central rib. The blocks provide the traction for the cross-country applications and the central rib provides favourable traction and ride for highway applications. However, because these prior art tread designs are a compromise between cross-country and highway uses they are not as efficient in off-the-highway applications as they might be.
Accordingly there is provided a pneumatic tire having a tread portion with a pair of lateral edges which are spaced apart by a given tread width and comprising a zig-zag rib at the mid-circumference of the tire and having an amplitude of between 60-70% of the tread width and having side faces thereon each of which is directed in one of the two rotational directions of the tire, the side faces each having a length of at least 40% of the tread width, and blocks located in the edge portions of the tread between adjacent peaks on the zig-zag rib.
The long length of exposed side face on the central rib provides good off-road traction, whilst the high amplitude zig-zag rib provides for a good highway ride.
Preferably said side faces on the rib are first side faces directed in one rotational direction, and second side faces directed in the opposite rotational direction, and the blocks each have two faces thereon each of which is substantially parallel to a respective one of the first and second sides faces on the rib and forming therebetween a first and second groove respectively, the width of each first groove being between 10-15% of the tread width, and the width of each second groove being between 35-100% of the width of the first groove.
By varying the width of the first grooves and the second grooves it is possible to obtain different degrees of traction for the two directions of rotation of the tire.